1. Field of the Invention
The present invention relates to a control method for an antiskid braking system for preventing wheels of an automobile from being locked when the automobile is braked, and more particularly, to a control method for an antiskid braking system adapted for use in a four-wheel-drive automobile.
2. Description of the Related Art
An antiskid braking system is used as a braking system for modern automobiles. This system is used to securely prevent wheels of an automobile from skidding or locking when the automobile is braked. Thus, the use of this system not only ensures the driving stability of the automobile, but also allows the braking distance to be shortened.
Conventional antiskid braking systems of this type include a so-called three-channel system. When the automobile is braked, according to the three-channel braking system, braking pressures on left and right front wheels of an automobile are controlled independently of each other in accordance with a tendency of each front wheel to skid, while braking pressures on left and right rear wheels are controlled together in accordance with a tendency of one of the rear wheels to skid. More specifically, the braking pressures on the rear wheels are controlled in accordance with the rotating behaviour of that rear wheel which more positively tends to skid.
When the three-channel antiskid braking system is applied to a four-wheel-drive automobile, the front and rear wheel sides are directly connected by means of a drive system. If the braking pressures on the front and rear wheel sides are independently controlled when the automobile is braked, therefore, a torsional torque in the drive system connecting the front and rear wheel sides increases. As a result, the speeds of the front and rear wheels are subject to undesired fluctuations, so that the braking pressures on the individual wheels cannot be controlled suitably and stably. The fluctuations of the wheel speeds induces vibrations of the body of the automobile, thereby making the automobile uncomfortable to ride in.
Conventionally, therefore, it is not advisable to apply the three-channel antiskid braking system to four-wheel-drive automobiles. Thus, if the three-channel antiskid braking system is incorporated in a part-time four-wheel-drive automobile, its operation may be prevented while the automobile is running in the four-wheel-drive mode, or otherwise, the drive mode of the automobile may be compulsorily switched from a four-wheel-drive mode to a two-wheel-drive mode.